Doc: createObject: adapted description of example

Plus language/style edits

Restored brief statement QQmlComponent.cpp

Task-number: QTBUG-32222

Change-Id: I07da54bad3e13f163475ed2a0760e66d50795f6f
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>